The present invention relates to an improvement in an STN (Super-Twisted Nematic) type liquid crystal display device which has liquid crystal molecules having a positive dielectric anisotropy that are twisted at least 90 degrees in a perpendicular direction to a display surface of the device.
A mode of display of a liquid crystal (hereinafter referred to as "L.C.") currently in use is the Twisted Nematic (hereinafter referred to as "TN") type which has L.C. molecules twisted about 90 degrees between a pair of polarizers and utilizes rotation of the polarization surface by the L.C. and a compensation of the rotation induced by a voltage. Although this mode can be applied to an instrument requiring only a low multiplexing drive, such as a watch and a portable electronic calculator, when it is applied to an instrument of a highly multiplexing drive in order to increase display capacity, disadvantages occur. The contrast is reduced and the display viewing angle is narrowed. These disadvantages occur because the ratio of working voltages on a selective point and a nonselective point approach 1.
To avoid these disadvantages, it is preferable to make a steepness, .gamma., as small as possible. The steepness, .gamma., is defined by the equation: EQU .gamma.=V.sub.50 /V.sub.10
wherein V.sub.10 represents a voltage difference with which a relative transmittance of the display changes 10% and V.sub.50 represents the same voltage except that the change is 50% instead of 10%.
The steepness of the TN type display is around 1.13. On the other hand, a display of STN or SBE (Super-Twisted Birefringence Effect) mode can make the steepness lower than 1.1 by making a twisted angle of L.C. molecule large and shifting polarizing axis from an aligned direction of L.C. molecules. As a result, it is possible to obtain a display of a highly multiplexing drive having a duty of around 1/400.
However, in these cases, a display surface gets colored when a field is applied because of existence of birefringence. Since it is necessary to utilize changes of the color according to the field strength for the display, there exists the disadvantage of poor contrast because a black and white display is very difficult in principle.
Japanese Patent Application, Laid-Open (KOKAI) 64-519 (1989) discloses a method to use a birefringent medium as a color compensation plate in order to avoid the disadvantages of STN type L.C. display device caused by the coloring. However, this method has another disadvantage, brightness and a color on display surface change according to a viewing direction.
The present inventors have studied extensively aiming to reduce disadvantages of the conventional STN type L.C. display device and to provide L.C. display device having high contrast throughout a wide range of viewing angle. Finally, they have found that by designing a L.C. display device comprising the following features, several advantages are realized:
A liquid crystal cell of the invention comprises:
(a) a pair of substrates having transparent electrodes and aligning films and a L.C. composition, with a positive dielectric anisotropy, positioned in a layer between the substrates, long axes of the L.C. molecules are aligned nearly homogeneous to a plane parallel to the substrate in the absence of an applied field and the molecules are twisted in an angle of 120.degree. to 360.degree. in a direction perpendicular to the substrate;
(b) a pair of polarizers are positioned on opposite sides of the cell; and
(c) one or more birefringent layers are positioned between the cell and at least one of the polarizers, a maximum refractive index direction at a plane of a layer contacting the polarizer and the maximum refractive index direction at a plane of a layer contacting the L.C. cell are tilted with respect to the substrate and angles made by the both maximum refractive index directions and with respect to the substrate are symmetrically aligned with each other.